Why doesn't git allow me to safely delete a branch which has been merged into HEAD?

I needed to merge two branches --

second

into

first

and then get rid of

second

. Here's what I did:

git clone

d the project to get a fresh copy

git checkout --track origin/second

, made some changes, and committed

git checkout --track origin/first

, made some changes, and committed

git merge second

(git says "merge made by recursive")

git branch -d second

Then git says:

$ git branch -d second
warning: not deleting branch 'second' that is not yet merged to
'refs/remotes/origin/second', even though it is merged to HEAD.
error: The branch 'second' is not fully merged.
If you are sure you want to delete it, run 'git branch -D second'.

Why is this happening? I've never gotten this message after a merge before. The merge worked just fine, no conflicts. How do I safely delete the